1. Field of the Invention
This invention, generally relates to analog-to-digital converters (ADCs) and, more particularly, to a system and method for correcting errors in an n-path interleaved ADC.
2. Description of the Related Art
An N-path time interleaved ADC consists of n component ADCs operated in parallel, which together sample the signal n times at the rate of the individual ADC components. In practice, the component ADCs are never truly identical, and the sampling clocks they receive can have small phase deviations from the ideal sampling phase. As a result, these timing and gain errors produce artifacts that in frequency domain show up as spectral images of the desired signal centered around every multiple of fs/N, where fs is the sampling rate of the composite ADC. If the errors are known they can be corrected with either digital post-processing after the ADC, or with an analog correction circuitry in the ADC, or with some combination of the two. However, without knowing the ADC input signal, error detection is difficult.
One way to facilitate the error correction task is to inject a narrow band known test signal into the ADC input, in the background, while the ADC is operating normally, as described in parent application Ser. No. 14/511,206, which is incorporated herein by reference. This method works well a two-path case where the test tone produces an image tone, which is also out-of-band and possible to detect with good accuracy, in an n-path time interleaved ADC, one test tone produces (n−1) image tones, which all need to be accurately estimated to extract all the information needed for error calibration. The problem is that only one of the tones is in a frequency region that is free of signal content. All the other image tones share the same frequencies as the ADC input signal (see FIGS. 6A and 6B) and are impractical to be detected with sufficient accuracy in the background while the ADC input signal is present.
It would be advantageous if error detection could be performed at a single out-of-band frequency location for all the mismatch parameters in an n-path time-interleaved ADC.